This invention relates to oscillator ballast circuits and is particularly suited to use with 35 and 40-watt fluorescent lamps.
Generally, fluorescent lamp ballast circuits in presentday use are of the auto-transformer type whereby the lamp is rendered non-conductive at a 120-Hz rate. Since auto-transformers are notoriously heavy, inefficient, and expensive, it can readily be seen that such apparatus is undesirable in view of the energy waste, excessive heat generated, and cost. Moreover, the non-conductivity rate of 120 Hz is well within the audio range and tends to provide undesired audible noise.
Another form of lamp ballast circuitry employs a flip-flop type oscillator circuit in cooperation with a saturable core transformer. A transistor of the oscillator saturates and effects saturation of the transformer core material to limit current flow and inhibit lamp burnout. However, core material saturation characteristics are erratic and unpredictable which renders such circuitry undesirable or at best, most difficult to predict or control.
In still another known form of ballast circuitry, an oscillator circuit is combined with a series resonant inductor and capacitor. Another inductor is placed in series connection with the series resonant circuit and provides drive potentials for the oscillator. Moreover, a load circuit is shunted across the capacitor.
Although the above-described ballast circuit has numerous advantages over other known techniques, it has been found that there are deficiencies which tend to inhibit its use. For example, it has been found that increased current flow through the inductor driving the transistors occurs when the load is removed and this increased current flow causes failure of the transistors of the oscillator circuit. Also, short circuiting the load circuit tends to cause failure of the oscillator transistors. Moreover, several transformers are required and no provision is made for tuning the oscillator circuitry to effect switching of the transistors at a minimum or zero current level whereby power losses are reduced. Also, no provisions are made for power factor correction.